Image forming apparatus and method having gradation control in a dense area in which gradation characteristics are non-linear

ABSTRACT

An image forming apparatus performs excellent gradation control in a dense area in which gradation characteristics are non-linear. The image forming apparatus has a pattern forming unit which forms a gradation pattern on a recording member, and a read unit which reads the gradation pattern formed by the pattern forming unit. An adjusting unit adjusts image forming conditions on the basis of the gradation pattern read by the pattern read unit. In a density area in which gradation characteristics are non-linear, the number of steps of the gradation pattern is set to be larger than that in another density area.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to image forming method and apparatus for formingan image on a recording material.

2. Related Background

Hitherto, there has been known a method whereby an image formingapparatus is activated and, after completion of a warm-up, a specificpattern is formed on an image holding member, a density of the patternon the image holding member is read and is fed back to produce imageforming conditions such as a gamma correction or calibration and thelike, thereby improving stability and image quality.

Further, in the case where image forming characteristics are changed dueto an environmental fluctuation or the like, the specific pattern isagain formed on the image holding member in accordance with anenvironmental fluctuation amount, a density of the pattern on the imageholding member is read and is fed back to produce the image formingconditions such as gamma correction or calibration and the like, therebystabilizing image quality.

In the above conventional method, however, in the case where the imageforming apparatus is used for a long time, there occurs a case where theread density of the pattern on the image holding member doesn't coincidewith a density of the image which was actually printed out.

For example, since a cleaning blade for cleaning a transfer residualtoner has come into contact with the image holding member and the imageholding member is rubbed for a long time, the surface of the imageholding member becomes rough and the relation between a depositionamount of the toner and a reflected light amount changes from an initialstate.

There is, consequently, a drawback such that when the image formingapparatus which was used for a long time is fed back to produce theimage forming conditions by using the density data converted by aninitial density conversion parameter, the optimum image cannot beobtained.

On the other hand, in the above conventional method, since noconsideration is made relative to a deterioration of the maximum imagedensity of the image forming apparatus, in the case where the maximumimage density output is decreased due to an influence by a durabilityfluctuation or the like, there is a drawback such that even if a gammacharacteristic is corrected, a gradation of the image deteriorates in aregion of a high image density.

In the above conventional method, since a gradation characteristic ofthe image forming apparatus is not linear (particularly, a highlight),when data between density data is interpolated by an approximateexpression, the resultant density differs from the actual density. Thereis, consequently, a drawback such that when the density is fed back toproduce the image forming conditions by using the gradation data, theoptimum image is not obtained.

Further, according to the above conventional method, when a uniformdensity is output to the whole surface of the recording member in theimage forming apparatus, in the case where a jump of a density appearsdue to a charging variation caused by dirt of a charging device, thedensity changes in dependence on the location even in case of the samedensity output. There is a drawback such that in the above state, if thedensity is fed back to produce the image forming conditions by using thegradation data, the optimum image is not obtained.

SUMMARY OF THE INVENTION

It is, therefore, an object of the invention to stabilize a picturequality by a plurality of kinds of calibrations.

Another object of the invention is to improve an operability duringcalibration.

Still another object of the invention is to effectively use the maximumdensity which can be expressed by an image forming means.

Further another object of the invention is to perform a good gradationcontrol in a density region in which a gradation characteristic is notlinear.

Further another object of the invention is to stably adjust imageforming conditions by judging the uniformity of a reference pattern.

According to the present invention, the above objects are accomplishedby an image forming apparatus comprising: input means for inputtingimage data; generating means for generating gradation pattern data;forming means for forming an image on a medium in accordance with theimage data or the gradation pattern data; detecting means for detectinga gradation pattern on the medium formed by the forming means inaccordance with the gradation pattern data; and control means forcontrolling a condition of the image forming apparatus based on thedetection result of the detecting means, wherein the gradation patterncomprises a plurality of areas of which density levels are differentfrom each other and a number of density levels are smaller in a densityrange in which a gradation characteristic is substantially more linearthan in another density range.

According to the invention, the above objects are also accomplished byan image forming method comprising: an input step of inputting imagedata; a generating step of generating gradation pattern data; a formingstep of forming an image or a medium in accordance with the image dataor the gradation pattern data; a detecting step of detecting a gradationpattern on the medium formed in the forming step in accordance with thegradation pattern data; and a control step of controlling a condition ofan image forming apparatus based on the detection result of thedetecting step, wherein the gradation pattern comprises a plurality ofareas of which density levels are different from each other and a numberof density levels are smaller in a density range in which a gradationcharacteristic is substantially more linear than in another densityrange.

The above and other objects and features of the present invention willbecome apparent from the following detailed description and the appendedclaims with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a constructional cross sectional view of an embodiment 1;

FIG. 2 which is comprised of FIGS. 2A and 2B is a constructional blockdiagram of a reader image processing unit 108 in the embodiment 1;

FIG. 3 is a diagram showing a timing of the reader image processing unit108 in the embodiment 1;

FIG. 4 is a control block diagram of the embodiment 1;

FIG. 5 is a block diagram showing the embodiment 1;

FIG. 6 is a 4-quadrant chart showing gradation reproducingcharacteristics;

FIG. 7 is a flowchart for a first control system;

FIGS. 8A to 8C are diagrams showing display contents of a display 218;

FIGS. 9A to 9C are diagrams showing display contents of the display 218;

FIGS. 10A to 10E are diagrams showing display contents of the display218;

FIG. 11 is a diagram showing an example of a test print 1;

FIG. 12 is a diagram showing an example of a test print 2;

FIG. 13 is a diagram showing a placing method of the test print 1 on anoriginal support plate;

FIG. 14 is a diagram showing a placing method of the test print 2 on theoriginal support plate;

FIG. 15 is a diagram showing the relation between a relative drumsurface potential and an image density;

FIG. 16 is a diagram showing the relation between an absolute moistureamount and a contrast potential;

FIG. 17 is a diagram showing the relation between a grid potential and asurface potential;

FIG. 18 is a diagram showing read points of a patch pattern;

FIG. 19 is a diagram showing a read example of the test print 2;

FIG. 20 is a diagram showing a LUT corresponding to each moistureamount;

FIG. 21 is a flowchart from a photosensor 40 to a density conversion;

FIG. 22 is a spectral characteristic diagram of a yellow toner;

FIG. 23 is a spectral characteristic diagram of a magenta toner;

FIG. 24 is a spectral characteristic diagram of a cyan toner;

FIG. 25 is a spectral characteristic diagram of a black toner;

FIG. 26 is a diagram showing the relation between a photosensor outputand an image density;

FIG. 27 is a flowchart for a second control system;

FIG. 28 is a diagram showing a detection example according to the secondcontrol system;

FIG. 29 is a diagram showing a patch forming sequence in the secondcontrol system;

FIG. 30 is a diagram showing a durability characteristic change of adensity conversion table of a photosensor 40;

FIG. 31 is a diagram showing density converting characteristics;

FIG. 32 is a diagram showing an example of a patch;

FIG. 33 is a diagram showing an example of a patch;

FIG. 34 is a diagram showing a construction of an embodiment 2 of theinvention;

FIG. 35 is a flowchart for the embodiment 2;

FIG. 36 is a diagram showing a test print which was printed out in theembodiment 2;

FIG. 37 is an explanatory diagram of measurement points;

FIG. 38 is an explanatory diagram of measurement points;

FIG. 39 is a diagram showing a modification of the embodiment 2;

FIG. 40 is a diagram showing a modification of the embodiment 2;

FIG. 41 is a diagram showing a modification of the embodiment 2;

FIG. 42 is a flowchart for an embodiment 3;

FIG. 43 is a diagram showing a test print of the embodiment 3;

FIG. 44 is a flowchart for an embodiment 4;

FIG. 45 is a constructional diagram of a developer; and

FIG. 46 is a diagram showing a modification of a test print.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First embodiment

An embodiment of the invention will now be described in detailhereinbelow with reference to the drawings.

FIG. 1 shows a constructional diagram of the embodiment.

A full-color image forming method will be first explained.

An original 101 put on an original support plate glass 102 is irradiatedby a light source 103 and an image of the original is formed on a CCDsensor 105 through an optical system 104. The CCD sensor 105 producescolor component signals of red, green, and blue, and every line sensorby a group of CCD line sensors of red, green, and blue are arranged inthree lines.

Those read optical system units scan in the direction of the arrow,thereby converting the original to an electric signal data train forevery line.

A collision member 107 for colliding the position of the original andthereby preventing the original from being obliquely placed is arrangedon the original support plate glass 102. A reference white plate 106 fordeciding a white level of the CCD sensor 105 and for performing ashading in the thrust direction of the CCD sensor 105 is arranged on theoriginal support plate glass.

The image signal obtained by the CCD sensor 105 is processed by a readerimage processing unit 108. After that, it is sent to a printer unit Band is processed by a printer control unit or a printer image processingunit 109.

The image processing unit 108 will now be described.

FIGS. 2A and 2B are block diagrams showing a flow of the image signal inthe image processing unit 108 of a reader unit A according to theembodiment. As shown in the diagram, the image signals which are outputfrom the CCD sensor 105 are input to an analog signal processing circuit201, by which they are subjected to a gain adjustment and an offsetadjustment. After that, the processed signals are converted to 8-bitdigital image signals R1, G1, and B1, and each color signal is convertedby an A/D converter 202, respectively. Subsequently, those digital imagesignals are input to a shading correction unit 203 and are subjected toa well-known shading correction using a read signal of the referencewhite plate 106 for every color.

A clock generating unit 211 generates a clock of a one-pixel unit. Amain scan address counter 212 counts the number of clocks from the clockgenerating unit 211 and produces a pixel address output of one line. Adecoder 213 decodes a main scan address from the main scan addresscounter 212 and produces a CCD drive signal of a line unit such as ashift pulse, a reset pulse, or the like, a VE signal indicative of aneffective area in the read signal of one line from the CCD, and a linesync signal HSYNC. The main scan address counter 212 is cleared by theHSYNC signal and starts to count the main scan address of the next line.

Since the line sensors of the CCD sensor 105 are arranged so as to bespaced from each other by a predetermined distance, a spatial aberrationin the sub scan direction is corrected. Specifically, the signals of Rand G are line-delayed in the sub scan direction, thereby matching withthe B signal in the sub scan direction for the B signal.

An input masking unit 205 is used to convert a read color space that isdetermined by spectral characteristics of filters of R, G, and B of theCCD sensor, to a standard color space of NTSC and executes a matrixarithmetic operation as shown by the following equation. ##EQU1##

A light amount/density converting unit (LOG converting unit) 206 isconstructed by a lookup table ROM. Luminance signals of R4, G4, and B4are converted to density signals of CO, MO, and YO. A line delay memory207 delays image signals of CO, MO, and YO by only line delay amountsuntil judgment signals of UCR, FILTER, SEN, and the like which areformed from the R4, G4, and B4 signals by a black character judgingsection (not shown).

A masking & UCR circuit 208 extracts a black signal (Bk) by three inputprimary color signals of Y1, M1, and C1 and further executes arithmeticoperations for correcting a color turbidity of a recording color memberin the printer unit B and sequentially outputs signals of Y2, M2, C2,and Bk2 by a predetermined bit width (8 bits) each time a readingoperation is executed. That is, the masking & UCR circuit 208 outputsframe-sequential data of Y, M, C, and Bk.

A gamma correction circuit 209 executes a density correction in thereader unit A so as to match with ideal gradation characteristics of theprinter unit B. A space filter processing unit (output filter) 210executes an edge emphasis and a smoothing process.

Area-sequential image signals of M4, C4, Y4, and Bk4 processed asmentioned above are sent to the printer control unit 109 and a densityrecording by a PWM (pulse width modulation) is executed in the printerunit B.

Reference numeral 214 denotes a CPU to perform a control in the readerunit; 215 a RAM; 216 a ROM; and 217 a console unit having a display 218.

FIG. 3 is a diagram showing a timing for each control signal in theimage processing unit 108 shown in FIGS. 2A and 2B. In the diagram, aVSYNC signal is an image effective interval signal in the sub scandirection. In an interval of a logic "1", an image reading (scan) isexecuted and output signals of (C), (M), (Y), and (Bk) are sequentiallyformed. The VE signal is an image effective interval signal in the mainscan direction and has a timing of the main scan start position in theinterval of logic "1" and is mainly used for a line count control of aline delay. A CLOCK signal is a pixel sync signal and is used totransfer image data at a leading timing of "0"→"1".

The printer unit B will now be described.

In FIG. 1, a photosensitive drum 4 is uniformly charged by a primarycharging device 8.

Image data is converted to a laser beam through a laser driver and alaser light source 110 included in the printer image processing unit109. The laser beam is reflected by a polygon mirror 1 and a mirror 2and is irradiated onto the photosensitive drum 4 which was chargeduniformly.

The photosensitive drum 4 on which a latent image was formed by a scanof the laser beam is rotated in the direction of the arrow shown in thediagram.

A developing device 3 sequentially executes a development every color.

In the embodiment, a 2-component system is used as a developing systemand the developing devices 3 of colors are arranged from an upstreamposition around the photosensitive drum 4 in accordance with the orderof black (Bk), yellow (Y), cyan (C), and magenta (M). The developingdevice according to the image signal executes a developing operation ata timing to develop a latent image area formed on the photosensitivedrum.

On the other hand, a transfer paper 6 is wrapped around a transfer drum5 and the drum is rotated one by one in accordance with the order of M,C, Y, and Bk, so that the transfer drum 5 is rotated a total four timesand the toner image of each color is transferred onto the transfer paper6 in a multiplexing manner.

After completion of the transfer of the toner images, the transfer paper6 is separated from the transfer drum 5 and is fixed by a fixing rollerpair 7, so that a full-color image print is completed.

A surface potential sensor 12 is arranged on an upstream side of thedeveloping devices 3 of the photosensitive drum 4.

A cleaner 9 is provided to clean the transfer residual toner on thephotosensitive drum 4. An LED light source 10 (having a main wavelengthof about 960 nm) and a photodiode 11 for detecting a reflection lightamount of a toner patch pattern formed on the photosensitive drum 4,which will be explained hereinlater, are also provided.

FIG. 4 is a constructional block diagram of the image forming apparatusaccording to the embodiment.

The printer image processing unit 109 is constructed by a CPU 28, a ROM30, a RAM 32, a test pattern memory unit 31, a density conversioncircuit 42, and an LUT 25 and can communicate with the reader unit A anda printer engine unit 100.

In the printer engine unit 100, an optical reading apparatus 40comprising the LED 10 for irradiating near-infrared light and thephotodiode 11, the primary charging device 8, a laser 101, the surfacepotential sensor 12, and the developing device 3 which are arrangedaround the photosensitive drum 4 are controlled by a printer controlunit 109.

An environmental sensor 33 is provided to measure a moisture amount inthe air in the apparatus.

The surface potential sensor 12 is provided on the upstream side thanthe developing devices 3. A grid potential of the primary chargingdevice 8 and development biases of the developing devices 3 arecontrolled by the CPU 28 as will be explained hereinlater.

FIG. 5 shows an image signal processing circuit to obtain a gradationimage according to the embodiment.

A luminance signal of the image is obtained by the CCD sensor 105 and isconverted to area-sequential image signal by the reader image processingunit 108. Density characteristics of the image signal are converted bythe LUT 25 so that a density of an original image that is expressed bythe image signal to which gamma characteristics of the printer at thetime of initial setting were input coincides with a density of an outputimage.

A state in which a gradation is reconstructed is shown by a 4-quadrantchart in FIG. 6.

The I quadrant shows reading characteristics of the reader unit A forconverting the original density to the density signal. The II quadrantshows converting characteristics of the LUT 25 to convert the densitysignal to the laser output signal. The III quadrant shows recordingcharacteristics of the printer unit B for converting the laser outputsignal to the output density. The IV quadrant shows total gradationreconstructing characteristics of the image forming apparatus showingthe relation from the original density to the output density.

Since the gradation is processed by the 8-bit digital signal, the numberof gradations is set to 256.

In the image forming apparatus, in order to make the gradationcharacteristics of the IV quadrant linear, a non-linear portion of theprinter characteristics of the III quadrant is corrected by the LUT 25of the IV quadrant.

The LUT 25 is formed by an arithmetic operation result, which will beexplained hereinlater.

After completion of the density conversion by the LUT 25, the signal isconverted to the signal corresponding to a dot width by a pulse widthmodulation (PWM) circuit 26 and is sent to a laser driver 27 to on/offcontrol the laser.

In the embodiment, a gradation expressing method by a pulse widthmodulating process is used for all of the colors of Y, M, C, and Bk.

By the scan of the laser light source 110, a latent image having apredetermined gradation characteristic is formed on the photosensitivedrum 4 due to a change in dot area and is subjected to processes such asdevelopment, transfer, and fixing, so that a gradation image isreproduced.

When a test pattern is formed, a pattern generator 29 generates apattern on the basis of data stored in the test pattern memory unit 31.

(Gradation control [calibration] of a system including both of thereader/printer)

A first control system regarding a stabilization of image reconstructingcharacteristics of a system including both of the reader unit A and theprinter unit B will now be described.

First, a calibration of the printer unit B by using the reader unit Awill be described with reference to a flowchart of FIG. 7. The flow isrealized by the CPU 214 to control the reader unit A and the CPU 28 tocontrol the printer unit B.

When the operator depresses a mode setting button for an automaticgradation correction or calibration provided on the console unit 217, apresent control is started. In the embodiment, the display 218 isconstructed by a liquid crystal operation panel (touch panel display)with push sensors as shown in FIGS. 8A to 10E. A display to the display218 is controlled by the CPU 214.

In step S51, a print start button 81 of a test print 1 appears on thedisplay 218 (refer to FIG. 8A). By depressing the button 81, an image ofthe test print 1 shown in FIG. 11 is printed out by the printer unit B.

In this instance, the presence or absence of the paper to form the testprint 1 is judged by the CPU 214. When such a paper doesn't exist, analarm display as shown in FIG. 8B is performed.

When the test print 1 is formed, a contrast potential (which will beexplained hereinlater) in a standard state according to an environmentis registered as an initial value and is used.

The image forming apparatus used in the embodiment has a plurality ofpaper cassettes and a plurality of kinds of paper sizes such as B4, A3,A4, B5, and the like which can be selected.

However, as a print paper which is used in this control, in order toavoid an error due to a mistake about the vertical placement or lateralplacement in a subsequent reading operation, what is called a large-sizepaper is used. Namely, it is set so as to use papers of the sizes of B4,A3, 11×17, and LGR.

A belt-like pattern 61 due to a half-tone density of four colors of Y,M, C, and Bk is formed in the test pattern 1 in FIG. 11. The testpattern 1 is formed on the basis of the data from the pattern generator29.

To visually inspect the pattern 61, it is confirmed that a stripe-shapedabnormal image, a density variation, and a color variation don't exist.As for the pattern 61, a size in the main scan direction of the CCDsensor 105 is set so as to cover a patch pattern 62 and gradationpatterns 71 and 72 (FIG. 12) in the thrust direction.

In the case where an abnormality is recognized, the test print 1 isagain printed. When an abnormality is again recognized, a service man iscalled.

It is also possible to read the belt pattern 61 by the reader unit A andto automatically judge whether the subsequent control should be executedor not by density information in the thrust direction.

On the other hand, the pattern 62 indicates the maximum density patch ofeach color of Y, M, C, and Bk and uses 255 levels as a density signalvalue.

In step S52, the image of the test print 1 is placed onto the originalsupport plate glass 102 as shown in FIG. 13 and a read start button 91shown in FIG. 9A is depressed.

In this instance, a guidance display for the operator shown in FIG. 9Aappears.

FIG. 13 is a diagram when the original support plate is seen from theupper portion. A left upper wedge-shaped mark T is a mark for originalcollision of the original support plate. The message as mentioned aboveis displayed on the operation panel in a manner such that the beltpattern 61 comes on the collision mark T side and the front and backsurfaces are not erroneously set. By performing such operations, acontrol error due to an erroneous placement is prevented.

When the pattern 62 is read by the reader unit A, a display screen isgradually scanned from the collision mark T. Since the first density gappoint A is obtained at the corner of the pattern 61, a position of eachpatch of the pattern 62 is determined as relative coordinates from thecoordinates point and the density value of the pattern 62 is read.

The display as shown in FIG. 9B is performed during the readingoperation. When the reading operation cannot be performed because thedirection or position of the test print 1 is incorrect, a message asshown in FIG. 9C is displayed. The operator again places the test printand depresses a read key 92, thereby again reading the test print 1.

To convert from the RGB values obtained to the optical densities, thefollowing equations (2) are used. To match with the same values as thoseof a commercially available densitometer, they are adjusted by acorrection coefficient (k).

It is also possible to convert from luminance information of RGB todensity information of MCYBk by using a LUT.

    M=-km×log10(G/255)

    C=-kc×log10(R/255)

    Y=-ky×log10(B/255)

    Bk=-kbk×log10(G/255)                                 (2)

A method of correcting the maximum density from the density informationobtained will now be described.

FIG. 15 shows the relation between the relative drum surface potentialand the image density obtained by the above arithmetic operations.

In the case where a difference between a contrast potential used at thattime point, namely, a development bias potential and the surfacepotential of the photosensitive drum when a dot of the maximum level isprinted by using the laser beam after the photosensitive drum wasprimary-charged is equal to a maximum density D_(A) derived by thesetting of A, in a density area of the maximum density, in most of thecases, the image density linearly corresponds to the relative drumsurface potential as shown by a solid line L.

In the 2-component development system, in the case where the tonerdensity in the developing device fluctuates and decreases, there is alsoa case where non-linear characteristics are obtained in the density areaof the maximum density as shown by a broken line N.

Therefore, although the final target value of the maximum density is setto 1.6, "1.7" is set to a target value of a control to match the maximumdensity and a control amount is determined in consideration of a marginof 0.1.

A contrast potential B in this instance is obtained by using thefollowing equation (3).

    B=(A+Ka)×1.7/D.sub.A                                 (3)

where, Ka denotes a correction coefficient and it is desirable tooptimize the value in accordance with a kind of developing system.

Actually, in the electrophotographic system, if the contrast potential Ais not set in accordance with the environment, the image density doesn'tmatch. Therefore, the set value of the contrast potential A is changedas shown in FIG. 16 in accordance with an output of the environmentalsensor 33 to monitor a moisture amount in the apparatus as describedabove.

Therefore, as a method of correcting the contrast potential, acorrection coefficient Vcont.rate1 according to the following equationis preserved into a backup RAM as a method of correcting the contrastpotential.

    Vcont.rate1=B/A

The image forming apparatus monitors a change in environment (moistureamount) every 30 minutes. Each time the value of A is decided on thebasis of the detection result, A×Vcont.rate1 is calculated, therebyobtaining the contrast potential.

A method of obtaining a grid potential and a development bias potentialfrom the contrast potential will now be simply explained.

FIG. 17 shows the relation between the grid potential and thephotosensitive drum.

The grid potential is set to -200 V. A surface potential V_(L) when thelevel of the laser beam is set to the lowest level and the scan isperformed and a surface potential V_(H) when the level of the laser beamis set to the highest level are measured by the surface potential sensor12.

Similarly, V_(L) and V_(H) when the grid potential is set to -400 V aremeasured.

By interpolating and extrapolating the data of -200 V and the data of-400 V, the relation between the grid potential and the surfacepotential can be obtained.

Such a control to obtain the potential data is called a potentialmeasurement control.

A difference between V_(L) and Vbg (set to 100 V here) set so as not todeposit an overlap toner onto the image is provided and a developmentbias V_(DC) is set.

A contrast potential Vcont is a differential voltage between thedevelopment bias V_(DC) and V_(H). As mentioned above, as Vcont islarge, the maximum density can be set to a large value.

To set to the contrast potential B obtained by the calculation, whethera grid potential of which volts and a development bias potential ofwhich volts are necessary can be judged by calculations from therelation of FIG. 17.

In step S53 in FIG. 7, a contrast potential is obtained so that themaximum density is higher than the final target value by 0.1. The CPU 28sets the grid potential and development bias potential so as to obtainsuch a contrast potential.

In step S54, a check is made to see if the obtained contrast potentiallies within a control range or not. When it is out of the control range,it is judged that there is an abnormality in the developing device orthe like. An error flag is set so that a service man can easily findsuch an abnormality and he checks the developing device of the colorcorresponding to the abnormality, thereby allowing the error flag to beseen by the service man in a predetermined service mode.

In this instance, in case of such an abnormality, the contrast potentialis limited to a limit value in the control range and a correction orcalibration control is executed (S55), and the control is continued.

As mentioned above, the CPU 28 sets the grid potential and developmentbias potential so as to set the contract potential as obtained in stepS53.

FIG. 31 shows a density conversion characteristic diagram. A printercharacteristic diagram of the III quadrant is as shown by a solid line Jby the maximum density control to set the maximum density in theembodiment to a value that is slightly higher than the final targetvalue.

If such a control is not performed, there is also a possibility suchthat printer characteristics which don't reach 1.6 as shown by a brokenline H are obtained. In case of the characteristics of the broken lineH, even when the LUT 25 is set to any table, since the LUT 25 doesn'thave an ability to raise the maximum density, a density between thedensity D_(H) and 1.6 cannot be reconstructed.

If the density is set to a value which slightly exceeds the maximumdensity as shown by the solid line J, a density reconstruction area canbe certainly assured by the total gradation characteristics of the IVquadrant.

As shown in FIG. 10A, a print start button 150 of the image of the testprint 2 appears on the operation panel. By depressing the button 150,the image of the test print 2 of FIG. 12 is printed out (S56). A displayas shown in FIG. 10B is performed during the printing.

As shown in FIG. 12, the test print 2 is constructed by a group ofpatches of a total of 64 gradations of four columns and 16 rows of eachof the colors of Y, M, C, and Bk. Among total 256 gradations, as for 64gradations, the laser output level is preponderantly allocated to anarea of a low density. In contrast, the laser output level is thinnedout in an area of a high density. With such a method, the gradationcharacteristics in a highlight portion, particularly, can be preferablyadjusted.

In FIG. 12, reference numeral 71 denotes the patch of a resolution of200 lpi (lines/inch) and 72 indicates the patch of 400 lpi (lines/inch).To form the image of each resolution, in the pulse width modulationcircuit 26, such an image can be realized by preparing a plurality ofperiods of a triangular wave which are used for comparison with theimage data as a target to be processed.

In the image forming apparatus, a gradation image is formed at aresolution of 200 lpi and a diagram image such as a character or thelike is formed at a resolution of 400 lpi. Patterns of the samegradation level are output by such two kinds of resolutions. However, inthe case where the gradation characteristics largely differ due to adifference of the resolutions, it is more preferable to set theforegoing gradation level in accordance with the resolution.

The test print 2 is generated from the pattern generator 29 withoutmaking the LUT 25 function (the characteristics of the LUT 25 arelinearly set).

FIG. 14 is a schematic diagram when the output of the test print 2 isplaced on the original support plate glass 102 and is seen from an upperposition. The left upper wedge-shaped mark T is a mark for originalcollision of the original support plate. A message is displayed on theoperation panel in a manner such that the pattern of Bk is located onthe collision mark T side and that the front and back surfaces are noterroneously placed (FIG. 10C). With such a method, a control error dueto a mistake of placement can be prevented.

When reading the pattern by the reader unit A, the display screen isgradually scanned from the collision mark T and the first density gappoint B is obtained. Therefore, the position of each color patch isdetermined as relative coordinates from the coordinates points and isread (S57).

As points which are read per one patch (73 in FIG. 12), as shown in FIG.18, 16 read points (x) are set in the patch and an average of sixteensignal values obtained is calculated for every component of RGB. It isdesirable to optimize the number of points in accordance with thereading apparatus, image forming apparatus, or size of patch.

The RGB signal in which the values of 16 points are averaged every patchis converted to the density value by the foregoing converting method tothe optical density. The optical density is set to the output densityand the laser output level is plotted on the axis of abscissa, so that adiagram as shown in FIG. 19 is obtained.

Further, as shown on an axis of ordinate on the right side, a basedensity of the paper (in the embodiment, 0.08) is set to 0 level and1.60 set as a maximum density of the image forming apparatus isnormalized to 255 levels.

In the case where the data obtained is such that the density issingularly high as shown at a C point or is low as shown at a D point,there is a case where there is dirt on the original support plate glass102 or there is a defect in the test pattern. Therefore, an inclinationis limited and a correction or calibration is made so as to keep acontinuity in a data train. Specifically, when the inclination is 3 ormore, it is fixed to 3. When the inclination has a minus value, thedensity level is set to the same density level as the previous level.

As mentioned above, the contents of the LUT 25 can be easily formed bymerely replacing the coordinates from the density level in FIG. 19 tothe input level (axis of density signal in FIG. 6) and from the laseroutput level to the output level (axis of laser output signal in FIG.6). With respect to the density level which doesn't correspond to thepatch, the value is obtained by an interpolating arithmetic operation.

In this instance, limiting conditions are provided so as to set theoutput level to 0 level for the input level of 0 level.

The conversion contents formed as mentioned above in step S58 are setinto the LUT 25.

In this manner, the contrast potential control by the first controlsystem using the read apparatus is executed and the gamma conversiontable is formed. During the above process, a display as shown in FIG.10D is executed and after completion of the display, display contentsare displayed as shown in FIG. 10E.

A supplemental control for the gradation after completion of the controlby the first control system will now be described.

In the image forming apparatus used in the embodiment, even when anenvironment fluctuates due to the foregoing contrast potential control,the maximum density can be corrected. However, a correction orcalibration are also executed with respect to the gradation.

In a state in which the first control system is made invalid, data inthe LUT 25 shown in FIG. 20 of each environment is preserved in the ROM30 in correspondence to the case where the environment changes.

Moisture amount data when the control by the first control system isexecuted is preserved and a LUT.A on the ROM 30 corresponding to themoisture amount is obtained.

Each time the environment changes after that, an LUT.B on the ROM 30corresponding to the moisture amount at that time point is obtained. ALUT.1 obtained by the first control system is corrected by the followingequation by using (LUT.B-LUT.A).

    LUT.present=LUT.1+(LUT.B-LUT.A)                            (4)

By such a control, the image forming apparatus is constructed so as tohave linear characteristics for the density signal, so that a variationin density gradation characteristics of each machine is suppressed. Astandard state can be set.

By enabling such a control to be used by the general user, by executingthe above control as necessary at a time point when it is judged thatthe gradation characteristics of the image forming apparatusdeteriorate, the correction or calibration of the gradationcharacteristics including both of the reader and the printer can beeasily executed.

Further, the correction or calibration for the environmental fluctuationas mentioned above can also be properly performed.

The setting about valid/invalid of the first control system can beperformed by the service man at the console unit 217. At the time of theservice maintenance, by invalidating the first control system, the stateof the image forming apparatus can be judged.

In case of invalidating, the apparatus is constructed in a manner suchthat a standard (default) contrast potential of such a kind of imageforming apparatus and the gamma LUT 25 are accessed from the ROM 30 andset. In this instance, characteristics of the gamma LUT are linear(through).

By constructing as mentioned above, a degree of aberration of thecharacteristics from the standard state can be known at the time of aservice maintenance. The optimal maintenance can be efficientlyexecuted.

(Gradation control [calibration] of printer)

The second control system regarding the stabilization imagereconstructing characteristics of the sole printer unit B will now bedescribed. The second control system is automatically executed by theCPU 28.

FIG. 21 shows a processing circuit for processing a signal from aphotosensor 40 comprising the LED 10 and photodiode 11 which face thephotosensitive drum 4. A near-infrared light from the photosensitivedrum 4 which enters the photosensor 40 is converted to an electricsignal by the photosensor 40. The electric signal is supplied to an A/Dconversion circuit 41, by which an output voltage of 0 to 5 V isconverted to a digital signal of 0 to 255 levels. The digital signal isfurther converted to the density by the density conversion circuit 42.

Color toners of yellow, magenta, and cyan are used in the embodiment.The toner is formed by dispersing a color member of each color by usinga copolymer resin of the styrene system as a binder.

Spectral characteristics of the yellow, magenta, and cyan toners areshown in FIGS. 22 to 24 in accordance with this order. As shown in thosediagrams, a reflectance of 80% or more is obtained for the near-infraredlight (main wavelength is set to 960 nm). In the formation of thosecolor toner images, a 2-component development system that isadvantageous for color purity and permeability is used.

In the embodiment, although the black toner is based on the same2-component development system, carbon black is used as a color memberin order to express pure black. Therefore, as shown in FIG. 25, areflectance of the near-infrared light (main wavelength: 960 nm) isequal to about 10%.

The photosensitive drum 4 is an OPC drum and a reflectance of thenear-infrared light (main wavelength: 960 nm) is equal to about 40%. Solong as the reflectances are almost equal, a drum of the amorphoussilicon system or the like can also be used.

FIG. 26 shows the relation between the output of the photosensor 40 andthe output image density when the density on the photosensitive drum 4is changed step by step in accordance with an area gradation of eachcolor.

An output of the sensor 40 in a state in which no toner is deposited onthe photosensitive drum 4 is set to 2.5 V, namely, the 128 level.

As will be understood from FIG. 26, as an area coating ratio increasesand the image density rises for each of the toners of yellow, magenta,and cyan, the output of the photosensor 40 is larger than that of thesole photosensitive drum 4.

On the other hand, in case of the black toner, as an area coating ratioincreases and the image density increases, the output of the photosensor40 is smaller than that of the sole photosensitive drum 4. From thosecharacteristics, by having a table 42a for converting to the densitysignal, the density signal can be accurately read out for every colorfrom an output signal of an exclusive-use sensor for each color.

A flow of the second control system will now be described with referenceto FIG. 27. Such a control is realized by the CPU 28.

When a main power switch is turned on (step S201), if a temperature of afixing roller is equal to or lower than 150° C., the control by thesecond control system is executed (S202).

When a fixing temperature is equal to or lower than 150° C., thetemperature of the fixing roller is set to a predetermined temperatureand a laser temperature also reaches a temperature adjustment point. Inorder to set the foregoing contrast potential, a potential measurementcontrol to measure potential data is performed. Until a toner tribologybecomes stable, the developing device is shut down and the apparatuswaits until it enters a standby state (S203).

When the apparatus enters the standby state, a patch pattern of each ofthe colors of Y, M, C, and Bk is formed on the photosensitive drum andis detected by the photosensor 40 (S204).

As a laser output of the patch, the 128 level is used as a densitysignal (axis of density signal in FIG. 6) for each color. In thisinstance, when setting the contents of the LUT 25 and the contrastpotential, those obtained in the first control system are corrected andused by using the moisture amount at that time point.

In a state in which the first control system is invalid, the LUT 25registered in a ROM 80 and the contrast potential which were led fromthe moisture amount as a standard state are used.

A sequence to form patches onto the photosensitive drum 4 is executed asshown in FIG. 28.

First, a gradation patch of magenta is formed at the first rotation ofthe photosensitive drum. A gradation patch of cyan is subsequentlyformed. By repeating the above operations twice, every two samegradation patches are formed in one cycle with respect to each color.

In the embodiment, since the photosensitive drum 4 of a large diameteris used, in order to accurately and efficiently obtain the density datain a short time, the patches of the same color are formed at thepositions which face by an angle of 180° on the photosensitive drum inconsideration of an eccentricity of the photosensitive drum. A pluralityof sampling operations are executed with respect to the patch of thesame color, thereby obtaining an average.

By forming patches of different colors so as to sandwich theabove-mentioned patch, data of two colors is obtained per one cycle.

As mentioned above, data of four colors are obtained per two cycles anddensity values are obtained by using the density conversion table 42a inFIG. 26.

As mentioned above, in the second control system, for instance, a goodcontrol can be performed by forming a gradation pattern different fromthat of the first control system in consideration of the characteristicsof the printer such as an eccentricity of the photosensitive drum or thelike.

FIG. 29 shows the relation between the density signal and the output.

Since an output density has been controlled by the first control systemso as to be 128 at a density scale in which 1.6 is normalized to 255.Therefore, in the case where the measured result is deviated by only ΔDas shown at an E point, a density signal 128 is obtained by thefollowing equation (5).

    Vcont.correct=Vcont.present×128/(128+ΔD)       (5)

In a manner similar to the first control system, by having a correctioncoefficient

Vcont.rate2=Vcont.correct/Vcont.present even if the environmentfluctuates, a correction or calibration is executed on the basis of thecontrast potential according to the environment preserved in the ROM 30(S206).

After completion of the above control, a message "copy capable" isdisplayed on the foregoing operation panel and the apparatus is set tothe copy standby state (S207).

The control by the second control system is completed by the operationsas mentioned above.

In many cases, generally, the power source of the image formingapparatus is turned off at night and is turned on in the morning.Therefore, in many cases, the second control system is activated once aday.

On the other hand, it is not presumed that the calibration by the firstcontrol system is frequently executed because it is normally performedat the time of installation.

Therefore, the service man executes the calibration of the first controlsystem when installing the image forming apparatus. If no problem occursin the image, the characteristics are automatically maintained by thecalibration by the second control system in a short period of time. Onthe other hand, the characteristics which gradually changed for a longperiod of time are corrected by the calibration by the first controlsystem. In this manner, roles can be divided. Thus, the gradationcharacteristics can be maintained until the life of the image formingapparatus expires.

A construction of the photosensor 40 used in the second control systemwill now be described.

In dependence on the use durability of the image forming apparatus,there occurs a case where the density obtained by reading the pattern onthe photosensitive drum 4 by the photosensor 40 doesn't coincide withthe density of the image which was actually printed out.

For example, the cleaning blade to clean the transfer residual tonercomes into contact with the photosensitive drum 4 and rubs the drumsurface for a long time, so that the surface of the photosensitive drum4 becomes coarse and the scattered light components of thephotosensitive drum increase. Thus, the relation between the output ofthe photosensor 40 and the image density changes from the initial state.

FIG. 30 shows an example of the case for yellow.

A curve 140 shows characteristics in the initial state of thephotosensitive drum. A curve 141 shows characteristics after 20000images are formed.

There is a tendency such that even in the case of the same sensoroutput, the image density decreases.

There is a case where even when the foregoing control is executed in astate in which the relation between the sensor output and the imagedensity doesn't match, good gradation characteristics are not derived.

After completion of the operation of the first control system, patchesin the second control system are formed on the photosensitive drum at alevel (the 96 level is used in the embodiment) near the output densitylevel 128 at the level of the gradation patch in the first controlsystem and are detected by a reading sequence.

An F point is obtained from the correspondence between the density valuewhich was read by the first control system and the output of thephotosensor 40.

The curve 140 has been stored as a conversion table in the ROM 30. Adensity corresponding to the photosensor output at a G point is equal toD1. A density corresponding to the G point after durability is equal toD2. Therefore, by multiplying a ratio of D2/D1 for the curve 140,conversion characteristics in a durable state can be obtained and usedfor correction or calibration.

According to the embodiment as described above, in the image formingapparatus to form a monochromatic image or a color image onto therecording member, the apparatus comprises: image reading means forreading the original on the original support plate and digitizing it;control means for controlling the reading conditions in accordance witha change in reading conditions in the image reading means; means forforming the toner image on the image holding member on the basis of theimage information read by the image reading means; reading means foroptically reading the reflection density of the toner image formed;means for transferring the toner image on the image holding member ontothe recording member; and means for fixing the toner image on therecording member, wherein the image forming apparatus further includes:a first control system for forming at least one or more image patternsto judge the image characteristics, reading such an image pattern by theimage reading means, and controlling the image forming conditions on thebasis of the read data; and a second control system for forming at leastone or more image pattern toner images to judge image characteristicsonto the image holding member, optically reading the reflection densityof the toner image formed by the reading means, and controlling theimage forming conditions on the basis of the read data. Therefore, thereis an effect such that the gradation characteristics can be maintainedfor a long time.

Particularly, the calibration by the first control system which needs tobe performed by the operator is manually executed for a long period oftime. The calibration by the second control system is automaticallyexecuted. Thus, the characteristics of the printer which easilyfluctuate can be automatically stabilized.

According to the embodiment, there is provided the image formingapparatus comprising: means for reading an original on the originalsupport plate and digitizing; means for forming an image on the basis ofthe digital image signal; and means for forming at least one or moreimage patterns for judging the image characteristics, placing thepattern image after it was output onto the original support plate,reading the image information, and controlling the image formingconditions by the information, wherein in the first step, the imagepattern of the maximum image density of the image forming apparatus isformed, the formed recording image is placed onto the original supportplate of the reading apparatus and is read, and the image formingconditions are controlled so as to be slightly higher than the targetmaximum density of the image forming apparatus on the basis of the readimage information, and in the second step, the image pattern indicativeof the density gradation is formed, the recording image formed is put onthe original support plate of the reading apparatus and is read, and bycontrolling the image forming conditions on the basis of the read imageinformation so that the gradation characteristics are set to constantcharacteristics, so that there are effects such that the output densityrange of the image forming apparatus is always set to the same state andthe stable gradation characteristics in a range from the highlight to ashadow can be always set to the same state.

When the patch of the maximum density is read, the maximum density canbe also obtained as an average value of a plurality of points (forinstance, three points). In the case where there are density gradientsin the thrust direction and circumferential direction of thephotosensitive drum 4, a density difference among the patches occurs.Therefore, when the detected density difference is larger than the setlevel, it is judged that there are some abnormalities in the positionalprecision of the photosensitive drum, positional precision of theprimary charging device, positional precision of the developing device,and the like. A message to urge the inspection is displayed on thedisplay 218 as an error message and the control can be also interrupted.

A plurality of patches of the maximum density of respective colors areformed as shown in FIG. 32 and an average can also be obtained. In thetest print 2, the respective colors can also be arranged in a line inthe sub scan direction as shown in FIG. 33.

In the above embodiment, although the density information has beenobtained by using the equation (2), an output of the LOG conversioncircuit 206 can also be used as density information. In this case, whenthe yellow density is measured, the value in which a signal of itscomplementary color, namely, blue is logarithmically converted and used.In case of the magenta density, a green signal is used. In case of thecyan density, a red signal is used. In case of the black density,although any color can be used in principle, the green signal can beused in consideration of spectral luminous efficiency characteristics.

In the above example, although Vcont has been corrected and calibratedby the second control system, by providing one more LUT similar to theLUT 25, the gamma correction or calibration can be also controlled bythe second control system. As mentioned above, by providing a pluralityof gamma correction table, the gradation characteristics of the printerwhich fluctuated by the use can be automatically adjusted in a shortperiod of time.

A target of the adjustment by the second control system is not limitedto Vcont but may be another processing condition such as driving time orthe like of a hopper screw in the toner supply control in the developingdevice.

Second Embodiment

FIG. 34 shows a construction of an image forming apparatus of the secondembodiment. In the embodiment, an image signal is converted to a laserbeam through a laser driver and a laser light source (not shown). Thelaser beam is reflected by a polygon mirror 1001 and a mirror 1002 andis irradiated onto a photosensitive drum 1004. The photosensitive drum1004 on which a latent image was formed by scanning the laser beam isrotated in the direction of the arrow shown in the diagram. Thus, adevelopment of each color is executed by a rotary developing device 1003(FIG. 34 shows the development by the yellow toner).

On the other hand, a recording member is wrapped around a transfer drum1005 and each time the drum is rotated once, the recording member isalso rotated one by one in accordance with the order of M (magenta), C(cyan), Y (yellow), and Bk (black) and the toner image is transferred tothe recording member from the photosensitive drum 1004 developed by therotary developing device 1003. The transfer operation is finished afterthe drum 1005 is rotated four times.

After completion of the transfer, the transfer paper is separated fromthe transfer drum 1005 and is fixed by a fixing roller pair 1007 and acolor image print is completed.

The second embodiment differs from the foregoing embodiment with respectto a point that the rotary developing device is used.

Since a circuit construction for image processes is similar to that inthe foregoing embodiment, its description is omitted.

FIG. 35 shows a flow for the embodiment. When a specific color in whichit was judged that there is an abnormality in the gradationcharacteristics is designated and a start switch of the control isturned on the operation panel 217 (S1001), as shown in FIG. 36, agradation pattern image having a large amount of portion in which thegamma characteristics of the designated color are not linear (having asmall step width) is formed on the recording member and is printed out(S1002). After that, the processes in steps S1003 to S1005 are executedin a manner similar to the foregoing embodiment.

As shown in FIG. 37, when a portion in which the gamma characteristicsare not linear is interpolated, the density differs from the actualdensity as shown by a broken line in the diagram. Therefore, in theembodiment, the portion in which the gamma characteristics are notlinear is densely eliminated, thereby reducing the difference with theactual density (FIG. 38). That is, in a density range in which thegradation characteristics are substantially more linear, the number ofpatches is reduced more than that of the other density range.

In the embodiment, when the LUT is formed, a primary interpolation hasbeen performed in order to produce the LUT data during such a period oftime. However, in order to improve precision, it is preferable toexecute a higher-order interpolation or a higher-order approximation.The LUT 25 is calculated and set by using the resultant data and thegradation performance can be improved.

By executing the above control, the number of steps of the patches canbe set to a large value in the density range in which the gradationcharacteristics (γ) are non-linear. Therefore, even in the portion inwhich the γ characteristics are non-linear, an image of an excellentgradation performance can be formed. That is, when the gradationpatterns of the same patch number are formed and the calibration isperformed, many patches can be allocated to the density range in which achange in gradation characteristics is large, so that the calibration ofa high precision can be executed.

In the above example, although the control has been performed in thedesignated single color, a deterioration in gradation performance due tothe durability can also occur with respect to all of the kinds of tonersused. Therefore, with regard to all of the colors of yellow, magenta,cyan, and black, the calibration can also be executed together.

FIG. 39 shows a flowchart. When the start switch for the control isturned on from the operation panel 217 (S1001), as shown in FIG. 40, apattern image having a number of gradation patterns of the portionswhere the γ characteristics of all colors are non-linear is formed onthe recording member by a pattern generator in the machine and isprinted out in a manner similar to the above control.

Since the gradation characteristics also fluctuate by the environment orprint output method, the embodiment is characterized in that the portionto be densely extracted is changed in accordance with the circumstances.In the first embodiment, gradation characteristics change in the caseswhere the pattern is output at a resolution of 200 lpi and where thepattern is output at a resolution of 400 lpi.

As mentioned above, in the case where the gradation characteristicschange in dependence on the output resolution, the measurement point ischanged as shown in FIG. 41 in accordance with them. By the measurementpoint, a control similar to the first embodiment is executed and animage of excellent gradation performance can be formed.

According to the embodiment as described above, the apparatus comprises:means for reading an original on the original support plate anddigitizing it; means for forming the toner image on the image holdingmember; means for transferring the toner image on the image holdingmember onto the recording member; and means for fixing the toner imageon the recording member, wherein the image pattern having many gradationpatterns of at least one or more portions in which the γ characteristicsare non-linear for judging the image characteristics is formed in therecording image after fixing, the image pattern is placed on theoriginal support plate and read, and the density adjustment is executedby using the data, thereby enabling the picture quality of a moreexcellent gradation performance to be obtained.

Third embodiment

A construction of the image forming apparatus of the third embodiment issimilar to that of the second embodiment.

FIG. 42 shows a flowchart of the embodiment. When the specific colorwhich was judged such that there is an abnormality in the gradationcharacteristics is designated and the start switch for the control isturned on, on the operation panel 217 (S2001), a half-tone image inwhich a density of the uniform density of the whole surface of thespecific output of the designated color is equal to near 0.6 isgenerated by the pattern generator in the machine. A print-out sample101' is again placed on the original support plate 102 of the reader andis irradiated by the light source 103. The reflected light istransmitted through the color separation optical system 104 and isconverted to the reflection light amount signal by the CCD sensor 105(S2003).

The reflection light amount signal is converted to the density data bythe logarithm conversion (S2004).

The density of the whole region of the half-tone image is judged (onepoint is measured in one millimeter square). When it is out of a rangeof a certain predetermined value (0.45 to 0.75) or when there is adifference of a predetermined value (0.15) or more between the maximumdensity and the minimum density, a message "Call service man" isdisplayed on the display 218 (S2007). After the abnormality of themachine was corrected, the gradation correction or calibration is againperformed. When the result of the judgment indicates the set value, asshown in FIG. 43, the pattern image of 256 gradations of the designatedcolor is formed on the recording member and is printed out. A gradationcontrol (S2006) based on the pattern is similar to the first embodiment.

In the embodiment, the density data of 256 points is used and the dataof the LUT 25 is calculated and set, thereby improving the gradationperformance.

By executing the above control, in the case where the density partiallychanges due to some inconvenience even in the same density output, theerroneous operation of the gradation correction or calibration can beprevented.

Fourth embodiment

In the above third embodiment, when some abnormality occurs in theapparatus, the user is merely urged to call the service man. In thefourth embodiment, however, a position where the abnormality occurs isalso notified.

The setting of the LUT 25 in the fourth embodiment is shown in FIG. 42in a manner similar to the third embodiment, an explanation of the sameprocesses as those in the third embodiment is omitted here.

In the fourth embodiment, when a check is made to see if there is anabnormality in the relation between the laser output level and the readdensity in step S2005 in FIG. 44, data such as a T/C ratio (ratio of thetoner to the carrier) and the like is also simultaneously referenced. Amethod of obtaining the T/C ratio will now be described hereinbelow withreference to FIG. 45.

FIG. 45 is a diagram showing a detailed construction of the developingdevice 1003 in FIG. 34. In FIG. 45, the toner is developed by thedeveloping device 1003 so that the latent image formed on thephotosensitive drum 1004 can be visually observed. However, in theembodiment, a developing agent comprising two components of the carrierand the toner is used.

In order to preserve the proper radio of the image density to the tonerdensity (T/C ratio) in the developing agent, the T/C ratio has to beheldconstant.

The developing device 1003 has screws 91a and 91b to uniformly stir thedeveloping agent. The screw 91a conveys the developing agent by rotatingon the right side in the diagram. The screw 91b conveys the developingagent by rotating on the left side in the diagram. Thus, the developingagent is circulated in the developing device 1003.

Since the carrier in the developing agent has magnetism, it is drawn upin a mixed state of the carrier and toner by a magnet built in adevelopment sleeve 90 and is uniformly coated on the photosensitive drum1004 by a blade 94. The toner of an amount corresponding to a differencebetween a voltage applied to the development sleeve 90 and a potentialof the latent image on the photosensitive drum 1004 is deposited on thephotosensitive drum 1004, so that the development is performed.

The developing device 1003 has therein an optical sensor comprising anLED 92 and a photodiode 93 in a state in which the optical sensor facesthe developing agent drawn up to the development sleeve 90.

Each of the LED 92 and photodiode 93 has a peak at a wavelength of 950nm. The toner which is reflected in such a wavelength range is used. Thecarrier which is absorbed in such a wavelength range is used. Therefore,when an output of the photodiode 93 is high, the T/C ratio is high. Onthe contrary, when the output of the photodiode 93 is low, the T/C ratiois low.

By previously storing the output of the photodiode 93 in a state of theset T/C ratio and by comparing with the present output of the photodiode93, a present T/C ratio can be detected by the a difference betweenthem.

When it is detected that the present T/C ratio is low, in order toreturn the T/C ratio to the set value, the toner is supplied into thedeveloping device 1003 from a toner supply unit (not shown).

The T/C ratio obtained as described above is referred when judgingwhether there is an abnormality in the relation between the laser outputlevel and the read density in step S2005 in FIG. 44.

In step S2005 in FIG. 44, when it is judged that there is an abnormalityin the relation between the laser output level and the read density,step S2100 follows and a message is displayed in the display 218. Inthis instance, a message to presume an abnormality occurrence positionon the basis of the T/C ratio obtained by the optical sensor in thedeveloping device 1003 mentioned above is displayed.

Specific examples of the messages to presume the above abnormalityoccurrence position will now be described hereinbelow.

For example, when considering that the read densities are set to threestages of "dense", "normal", and "light" and the T/C ratio is also setto three stages of "dense", "normal", and "light", nine states as shownin the following table 1 are considered.

                  TABLE 1                                                         ______________________________________                                                  Reader density                                                      T/C ratio dense         normal  light                                         ______________________________________                                        dense     1             2       3                                             normal    4             5       6                                             light     7             8       9                                             ______________________________________                                    

Messages for those states shown in the above table 1 can be set, forexample, as shown in the following table 2.

                  TABLE 2                                                         ______________________________________                                        State       Message                                                           ______________________________________                                        1           Until the T/C ratio decreases, please                                         continuously output a picture for a                                           little while                                                      2, 3, 6     Please examine developing device or                                           transfer relation                                                 4, 7, 8     Please examine developing device                                  5           Normal                                                            9           Please examine hopper                                             ______________________________________                                    

As described above, according to the fourth embodiment, when anabnormality such as an apparatus abnormality or the like occurs at thetime of gradation correction or calibration, it is detected and,further, the abnormality occurring position is presumed, so that anerroneous operation can be prevented and the maintenance can be easilyexecuted.

In the above embodiment 4, one half-tone image is output in order toconfirm the uniform density. If it is correct, the pattern for thegradation control is output. However, they can be also output as oneimage together.

That is, as shown in FIG. 46, at least one or more gradation imagepatterns for judging image characteristics are formed in a portion ofthe recording member and a uniform density half-tone pattern is outputfor the other portions.

With such a method, a troublesome operation to form a test pattern anumber of times can be omitted and an efficient calibration can beperformed.

As described above, according to the embodiment, the apparatus has meansfor reading the original on the original support plate and digitizing itand means for forming an image on the basis of the digital signal,wherein before performing the gradation correction or calibration, auniform output image is generated and is placed on the original supportplate and is read, and in the case where an abnormal portion in the readdata, a message "Call the service man" is displayed and no gradationcorrection or calibration is performed in this instance. Further, dataof a patch sensor (FIG. 45) or the like is monitored and in the casewhere although the T/C ratio is high, the density is thin, a messageindicating that the occurrence of an abnormality such as "1. Checkwhether there is an abnormality in developing device. 2. Check whetherthere is an abnormality in copy transfer. 3. Check whether there is anabnormality in hopper. 4. ******" or the like can be presumed is alsodisplayed. After the abnormality of the machine is repaired, thegradation correction or calibration is again executed. Thus, anerroneous operation of the gradation correction or calibration can beprevented and the maintenance can be easily executed.

As described above, although the embodiment has been described withrespect to the gradation control in the electrophotographic system as anexample, the invention can also be applied to an image forming apparatususing a printer of the ink jet system, particularly, what is called abubble jet system having a head of the type such that a liquid dropletis emitted by using film boiling by thermal energy.

The invention is not limited to the recording apparatus for representingmulti-gradation with respect to one pixel, and is also applicable to arecording apparatus for representing a binary value with regard to onepixel.

In the above example, although a feedback destination at the time of thecalibration has been set to the gamma correction or calibration, forinstance, the data can also be fed back to the masking & UCR of themasking & UCR circuit 208 in order to preferably correct a color mixtureof recording members of a plurality of colors.

The present invention is not limited to the foregoing embodiments butmany modifications and variations are possible within the spirit andscope of the appended claims of the invention.

What is claimed is:
 1. An image forming apparatus comprising:input meansfor inputting image data; generating means for generating gradationpattern data; forming means for forming an image on a medium inaccordance with the image data or the gradation pattern data; detectingmeans for detecting a gradation pattern on the medium formed by saidforming means in accordance with the gradation pattern; and controlmeans for controlling a condition of said image forming apparatus basedon the detection result of said detecting means; wherein said gradationpattern comprises a plurality of areas of which density levels aredifferent from each other and a number of density levels are smaller ina density range in which a gradation characteristic is substantiallymore linear than in another density range.
 2. An apparatus according toclaim 1, wherein said medium is a photosensitive member.
 3. An apparatusaccording to claim 1, wherein said medium is a paper.
 4. An apparatusaccording to claim 1, wherein said input means is a reading means forscanning an original and generating image data.
 5. An apparatusaccording to claim 1, wherein said generating means includes a ROM. 6.An apparatus according to claim 1, wherein said forming means forms alatent image onto a photosensitive member and develops the latent imageand forms an image onto the medium.
 7. An apparatus according to claim1, wherein said generating means generates a gradation pattern for eachcolor component.
 8. An apparatus according to claim 1, wherein saidgenerating means generates a plurality of gradation patterns of whichresolutions are different from each other.
 9. An apparatus according toclaim 8, wherein said number of density levels in a first gradationpattern is different from said number of density levels in a secondgradation pattern.
 10. An apparatus according to claim 1, wherein saidcontrol means controls an adjustment of the gradation characteristic ofsaid image data.
 11. An apparatus according to claim 10, wherein saidadjustment is performed by a lookup table.
 12. An image forming methodcomprising:an input step of inputting image data; a generating step ofgenerating gradation pattern data; a forming step of forming an image ona medium in accordance with the image data or the gradation patterndata; a detecting step of detecting a gradation pattern on the mediumformed in said forming step in accordance with the gradation patterndata; and a control step of controlling a condition of an image formingapparatus based on the detection result of said detecting step, whereinsaid gradation pattern comprises a plurality of areas of which densitylevels are different from each other and a number of density levels aresmaller in a density range in which a gradation characteristic issubstantially more linear than in another density range.
 13. An imageforming apparatus comprising:input means for inputting image data;generating means for generating a plurality of gradation pattern data;forming means for forming an image on a medium in accordance with theimage data or the gradation pattern data; detecting means for detectinga gradation pattern on the medium formed by said forming means inaccordance with the gradation pattern; and control means for controllinga condition of said image forming apparatus based on the detectionresult of said detecting means; wherein each such gradation patterncomprises a plurality of areas of which density levels are differentfrom each other and a number of density levels are smaller in a densityrange in which a gradation characteristic is substantially more linearthan in another density range, and the number of density levels in afirst gradation pattern is different from the number of density levelsin a second gradation pattern.
 14. An apparatus according to claim 13,wherein said medium is a photosensitive member.
 15. An apparatusaccording to claim 13, wherein said medium is a paper.
 16. An apparatusaccording to claim 13, wherein said input means is a reading means forscanning an original and generating image data.
 17. An apparatusaccording to claim 13, wherein said generating means includes a ROM. 18.An apparatus according to claim 13, wherein said forming means forms alatent image onto a photosensitive member and develops the latent imageand forms an image onto the medium.
 19. An apparatus according to claim13, wherein said generating means generates a gradation pattern for eachcolor component.
 20. An apparatus according to claim 13, wherein saidcontrol means controls an adjustment of the gradation characteristic ofsaid image data.
 21. An apparatus according to claim 20, wherein saidadjustment is performed by a lookup table.
 22. An image forming methodcomprising:an input step of inputting image data; a generating step ofgenerating a plurality of gradation pattern data; a forming step offorming an image on a medium in accordance with the image data or thegradation pattern data; a detecting step of detecting a gradationpattern on the medium formed in said forming step in accordance with thegradation pattern data; and a control step of controlling a condition ofan image forming apparatus based on the detection result of saiddetecting step, wherein each such gradation pattern comprises aplurality of areas of which density levels are different from each otherand a number of density levels are smaller in a density range in which agradation characteristic is substantially more linear than in anotherdensity range, and the number of density levels in a first gradationpattern is different from the number of density levels in a secondgradation pattern.